Prior to the use of supercritical fluids, such as supercritical carbon dioxide, to replace volatile solvents that cause air pollution, the liquid spray application of coatings, such as paints, lacquers, enamels, and varnishes, was effected through the use of organic solvents as viscosity reduction diluents. However, because of increased environmental concern, efforts have been directed to reducing the pollution resulting from coating operations.
While the utilization of supercritical fluids as viscosity reducing diluents in coating applications has met with much success and is effectively able to accomplish the primary objective of reducing the emission of volatile organic compounds into the atmosphere while simultaneously providing the stringent performance characteristics of the applied coating that is required by the coatings industry, a desire has nevertheless arisen to determine whether such viscosity reducing diluent effects can be obtained with suitable materials at conditions which are below the supercritical fluid state, i.e., with subcritical compressed fluids. In particular, this desire has been generated by the realization that there are coating materials which may contain components that are highly temperature sensitive, such as highly reactive cross-linking coating systems and two-package coating systems that are employed in high-solids coatings. As such, it may be undesirable to subject such components to the temperatures required to keep the viscosity reducing supercritical fluid in its supercritical state. By spraying at significantly lower temperature, it may be possible to spray a two-package coating using a conventional single-feed coating system instead of having to feed the two reactive components separately as required at elevated spray temperatures. In cross-linking coating systems, the undesirable increase in spray viscosity that results from premature reaction in the heated spray equipment can therefore be avoided or minimized.
Furthermore, it is common for sprays with supercritical fluids to be heated to relatively high temperatures, typically 50 to 60 C. or higher, to offset the cooling effect that occurs as the supercritical fluid expands from the spray as a free gas. This requires either the use of a circulating flow of the heated spray mixture, which is undesirable for spray operations that use color change or highly reactive coating systems, due to increased volume and residence time, or the use of a specially heated and thermostated spray gun and feed line in order to maintain the proper spray temperature at all times including at start-up. This increases the amount of equipment that must be operated and maintained in the spray operation, which increases equipment and labor costs and makes the operation more susceptible to interruptions due to equipment failure or loss of temperature control.
So too, generally depending on the particular coating composition to be sprayed, at the typically higher pressures needed to maintain the viscosity reducing supercritical fluid in its supercritical state, i.e., above its critical pressure, and at the typically still higher pressures required to obtain high solubility of the supercritical fluid at the elevated temperatures, more wear and tear may be experienced on the spray coating equipment, particularly the spray gun nozzle tips. Moreover, there is a general desire, particularly for safety reasons, to work with a process at a pressure which is as low as possible while still realizing the overall benefits of such a process.
Lower pressures also produce a lower velocity spray, which is advantageous for obtaining higher transfer efficiency in depositing the coating composition onto a substrate and in particular for making electrostatic deposition more effective. Lower spray velocity can also improve coating quality by reducing the amount of fine air bubbles that become entrapped in the liquid coating as the spray strikes the substrate, which may cause undesirable haze to occur in clear coatings and may promote solvent popping during baking. Lower pressures also make it possible to obtain lower spray application rates without having to use extremely small orifice sizes that are susceptible to becoming plugged and are difficult to manufacture. There is also a general desire to spray at the lower pressures at which air-assisted airless spraying is practiced, which are generally below supercritical fluid pressures.